In cryptography, M8 is a block cipher designed by Hitachi in 1999. It is a modification of Hitachi's earlier M6 algorithm, designed for greater security and high performance in both hardware and 32-bit software implementations.
Like M6, M8 is a 10-round Feistel cipher with a block size of 64 bits. The round function can include bit rotations, XORs, and modular addition, but the structure of each round function used is determined by the key. Making these variations key-dependent is intended to make cryptanalysis more difficult (see FROG for a similar design philosophy).